A solar battery is greatly expected to serve as a clean renewable energy source, and researches have been conducted for practical application of monocrystalline-silicon, polycrystalline-silicon or amorphous-silicon-based solar batteries and solar batteries comprising, for example, cadmium telluride or indium copper selenide. For the spread of solar battery as a household power source, however, any of these batteries faces many problems to be overcome, including a higher production cost, difficulty in ensuring raw materials, difficulty in recycling, and difficulty in realizing a larger area. Accordingly, there have been proposed solar batteries comprising an organic material in an attempt to achieve a larger area and a lower cost. However, any of these batteries has a conversion efficiency of about 1%, which falls very short of practical use.
Under such circumstances, Graetzel et al. disclosed a photoelectric conversion element and a solar battery comprising semiconductor particles sensitized by a dye, as well as materials and production technique needed to produce this solar battery, in 1991 (see, for example, Non-patent document 1 and Patent document 1). This battery is a wet solar battery comprising a porous titania thin film sensitized by a ruthenium dye as a working electrode. This solar battery has the advantages that the photoelectric conversion element can be provided at a low cost because inexpensive materials can be used without highly purification, and that the solar battery can convert solar light into electricity over a wide visible light wavelength range because the dye used in the solar battery has a broad absorption band. However, the conversion efficiency must be further improved for practical use. Thus, there is a need for development of a dye which has a higher absorption coefficient and absorb light of longer wavelength.
Patent document 2 discloses a mononuclear metal complex containing a dipyridyl ligand, which is a metal complex dye useful for a photoelectric conversion element. In addition, Non-patent document 2 discloses a polynuclear β-diketonate complex dye.
Meanwhile, Patent document 3 discloses a polynuclear complex containing a plurality of metals and a plurality of ligands wherein a bridging ligand (BL) coordinating to the plurality of metals has both a coordination structure with a conjugated heterocyclic ring and a coordination structure without a conjugated heterocyclic ring, which is regarded as a novel polynuclear complex having the excellent photoelectric conversion function of emitting electrons while receiving energy from active ray such as light.
In addition, Patent document 4 discloses a binuclear metal complex having a coordination structure with a conjugated heterocyclic ring, which is a metal complex dye for realizing a photoelectric conversion element having higher photoelectric conversion efficiency.